University of Birminghametheses.bham.ac.uk/id/eprint/8302/1/Contreras_Vasquez18PhD.pdfSYNOPSIS Synthesis, characterisation and hydrogen sorption properties of a variety of magnesium

MECHANICAL SYNTHESIS OF

MAGNESIUM ALLOYS FOR HYDROGEN

STORAGE

by

Luis Felipe Contreras Vásquez

A thesis submitted to

The University of Birmingham

For the degree of

DOCTOR OF PHILOSOPHY

School of Metallurgy and Materials

College of Engineering and Physical Sciences

University of Birmingham

September 2017

University of Birmingham Research Archive

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SYNOPSIS

Synthesis, characterisation and hydrogen sorption properties of a variety of magnesium based

hydrides were investigated in this work. The structure of these composites was studied using

X-ray diffraction (XRD) and Raman spectroscopy.

The thermal stability and decomposition reactions of the Mg-based hydrides was studied using

differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), coupled with

a mass spectrometer (MS) to determine the gaseous products released during heating.

Compositional changes and reversibility were investigated in detail using in-situ XRD under

Ar and H2.

Mechanical milling of magnesium hydride (MgH2) under Ar and H2 resulted in a reduction of

the crystallite size from 207 nm for the as received to 18 nm for 10 h milled MgH2. For the first

time was reported the effect of Raman spectroscopy on milled MgH2 demonstrating that milled

samples were Raman active. Hydrogen desorption temperatures were decreased ~120 °C (DSC)

with increasing milling time (10 h), however, hydrogen capacity was decreased ~6.55 wt. %

(TGA).

Hydrogenation of Li-Mg alloy was investigated via reactive milling in 100 bar H2, after 1 h

milling Li-Mg phase was hydrogenated forming LiH and MgH2. Hydrogen desorption was

observed at 250 °C (DSC), releasing 0.19 wt. % (TGA). Although, the desorption temperature

was decreased, the amount of hydrogen released is insignificant and is hard to consider for on-

board applications.

Mechanical milling of sodium hydride (NaH) and magnesium hydride (MgH2) under Ar and H2

lead to the formation of sodium magnesium ternary hydride (NaMgH3). Thermal decomposition

occurred at ~ 325 °C with a mass change of 5 wt. %, associated with the evolution of hydrogen.

Hydrogen desorption occurred in two-step reactions. Rehydrogenation of the NaMgH3 hydride

was observed at 250 °C and 10 bar H2.

Mechanical milling of lithium hydride (LiH) substituted into NaMgH3 hydride resulted in the

formation of a quaternary LixNa1-xMgH3 hydride with molar compositions (x=0, 0.2, 0.5, 0.8).

Thermal decomposition started at 250 °C, releasing a total amount of 5 wt% of H2.

Decomposition reactions occurred in two and three steps. Furthermore, reversibility of the main

phase was achieved at 250 °C and 10 bar H2.

Milling calcium hydride (CaH2) and MgH2 lead to the formation of calcium magnesium (Ca-

Mg-H) ternary hydride. Hydrogen sorption characteristics showed a dehydrogenation

temperature of 325 °C (DSC) with a total amount of 2.24 wt.% H2 evolution up to 500 °C.

However, dehydrogenation of CaH2 was not completed even at 500 °C. Thermal decomposition

suggested two steps reactions. Reversibility was successfully achieved at 365 °C and 10 bar H2.

LiH and NaH were substituted into the Ca-Mg-H to form quaternary hydrides with composition

MxCa1-xMgH4. Hydrogen sorption properties showed desorption temperatures between 300 °C

and 385°C with a maximum of 3.5 wt.% H2 released. Thermal decomposition proceeded in

three-step reaction. Nonetheless, complete dehydrogenation was not achieved.

Overall, this investigation has demonstrated for a variety of Mg-based hydrides that reducing

crystallite size has a positive effect in the sorption properties, unfortunately, none of the

materials and composites investigated in this work meet the targets for light-duty vehicles set

out by the Department of Energy (DoE). However, other applications such as heat storage might

be of interest.

ACKNOWLEDGEMENTS

First of all, I would like to thank Almighty GOD for the opportunity to be at this stage of my

life, successfully accomplishing one more goal, embarking into a new dream. Thank you for

the wisdom, perseverance, love and faith given through Jesus Christ and the Holy Spirit to walk

firmly and not tumble. All the Glory Must be to The Lord.

Special thanks dedicated to you my beloved wife, for your patience, love, support, and

sacrifices. For leaving everything and everyone behind, to help me to accomplish this degree. I

can certainly say that without you by my side, I would not be able to be where I am now. Thank

you for giving me the best gift one can dream of, a beautiful angel that completely changed my

perception of what life is. I love you both, and I am grateful for the time, love and laughs shared

with me during these years. I am blessed to have you.

Thank you to my family for the love, support, and prayers. You encouraged me to keep going

forward no matter what, backed me up as a family and showed me the importance of having

GOD in my life.

Thank you to my supervisor Professor David Book for his guidance and support on every stage

of this work and for the opportunity to be part of the Hydrogen group at the University of

Birmingham.

Thank you to Dr. Reed, Mr. Simon Cannon, Dr. Joshua Vines, Dr. Luke Hughes, Dr. Sheng

Guo and all my fellows at hydrogen group, for their help and share knowledge of the equipment.

Thank you for all the food shared during our social hours.

I also would like to extend my thanks to the Government of Ecuador through the SENESCYT

for the grant awarded to study my postgraduate degree at the University of Birmingham.

CONTENTS

1. INTRODUCTION ................................................................................................................ 1

1.1 Introduction ..................................................................................................................... 1

1.2 Hydrogen as Energy vector ............................................................................................. 3

1.3 Hydrogen Storage ............................................................................................................ 8

1.3.1 Physical-based storage ................................................................................................. 13

1.3.1.1 Compressed Gas Hydrogen Storage ...................................................................... 13

1.3.2 Material-based Storage ................................................................................................ 15

1.3.2.1 Physisorption ......................................................................................................... 15

1.3.2.3 Complex Hydrides................................................................................................. 16

1.3.2.4 Metal hydrides ....................................................................................................... 17

1.4 Summary ............................................................................................................................. 21

2. Mg-BASED HYDRIDES ..................................................................................................... 24

2.1 Introduction ........................................................................................................................ 24

2.2 Mg/MgH2 Structures ........................................................................................................... 25

2.3 Hydrogen Storage Properties .............................................................................................. 27

2.3.1 Kinetics ........................................................................................................................ 27

2.3.1.1 Effect of Microstructural Modification ................................................................. 27

2.3.1.2 Catalysis and additives .......................................................................................... 28

2.3.2 Thermodynamic behaviour .......................................................................................... 30

2.3.3 Ternary Hydrides ......................................................................................................... 34

2.3.3.1 Na-Mg-H (Sodium Magnesium Hydride) ............................................................. 35

2.3.3.2 Ca-Mg-H (Calcium Magnesium Hydrides)........................................................... 37

2.3.3.3 Li-Mg-H (Lithium Magnesium Hydrides) ............................................................ 38

2.3.4 Destabilisation of ternary hydrides by light-weight metals substitution. .................... 40

2.3.4.1 Li substitution into Na-Mg-H hydride .................................................................. 40

2.3.4.2 Li and Na substitution into Ca-Mg-H hydrides ..................................................... 42

2.4 Magnesium-based hydrides applications. ........................................................................... 42

2.5 Summary ............................................................................................................................. 44

2.6 Aims and Objectives ........................................................................................................... 46

3. EXPERIMENTAL METHODS ........................................................................................... 48

3.1 Introduction ........................................................................................................................ 48

3.2 Starting Materials ............................................................................................................... 49

3.3 Synthesis of the material..................................................................................................... 49

3.3.1 Mechanical milling ...................................................................................................... 50

3.4 Structural Characterisation ................................................................................................. 52

3.4.1 X-ray Diffraction ......................................................................................................... 52

3.4.1.1 Powder X-ray diffraction ...................................................................................... 54

3.4.1.2 In-Situ X-ray diffraction........................................................................................ 55

3.4.1.3 X-ray diffraction data analysis .............................................................................. 56

3.4.2 Raman Spectroscopy .................................................................................................... 57

3.4.3 Microscopy .................................................................................................................. 59

3.5 Thermal Decomposition ..................................................................................................... 61

3.5.1 Differential Scanning Calorimetry (DSC) ................................................................... 61

3.5.2 Thermogravimetric Analysis (TGA) ........................................................................... 62

3.5.3 Mass Spectrometry ...................................................................................................... 64

4. MAGNESIUM HYDRIDE................................................................................................... 66

4.1 Introduction ........................................................................................................................ 66

4.2 Characterisation of as-received and milled MgH2 .............................................................. 67

4.3 Thermal Decomposition ..................................................................................................... 86

4.4 Analysis of Decomposition Products ................................................................................. 92

4.5 Conclusions ........................................................................................................................ 95

5. RESULTS AND DISCUSSION: LITHIUM MAGNESIUM ALLOY ............................... 98

5.1 Introduction ........................................................................................................................ 98

5.2 Structural characterisation of as received and as milled Li-Mg alloy (ribbon) .................. 99

5.3 Thermal decomposition .................................................................................................... 104

5.3.1 DSC-TGA measurements .......................................................................................... 104

5.3.2 In-Situ XRD ............................................................................................................... 108

5.3.2.1 Li-Mg Ribbon (as-received) ................................................................................ 108

5.3.2.2 Li-Mg 1h milled in 100 bar H2. ........................................................................... 110

5.4 Conclusions ...................................................................................................................... 114

6. RESULTS AND DISCUSSION: SODIUM MAGNESIUM HYDRIDE .......................... 117

6.1 Introduction ...................................................................................................................... 117

6.2 Characterisation of as-received materials ......................................................................... 121

6.3 Characterisation of as-milled materials ............................................................................ 123

6.4 Thermal decomposition .................................................................................................... 129

6.5 Analysis of Products after Dehydrogenation .................................................................... 133

6.5.1 In-situ XRD of milled samples in Ar ......................................................................... 133

6.5.2 In-situ XRD of milled samples in 100 bar H2 ............................................................ 137

6.6 Reversibility. .................................................................................................................... 140

6.7 Conclusions ...................................................................................................................... 146

7. RESULTS AND DISCUSSION: LixNa1-xMgH3 HYDRIDE SYSTEM ............................ 149

7.1 Introduction ...................................................................................................................... 149

7.2 Sample Preparation ........................................................................................................... 150

7.3 Characterisation of milled material .................................................................................. 151

7.4 Measurement of hydrogen storage capacity of the LixNa1-xMgH3 (x=0, 0.2, 0.5, 0.8)

hydrides .................................................................................................................................. 155

7.4.1 Thermal analysis (DSC-TGA-MS) ............................................................................ 155

7.4.2 Analysis of Decomposition Products (in-situ XRD) ................................................. 159

7.5 Recombination ability ...................................................................................................... 177

7.5.1 In-situ XRD ................................................................................................................ 177

7.6 Conclusions ...................................................................................................................... 193

8. RESULTS AND DISCUSSION: CALCIUM MAGNESIUM HYDRIDE ....................... 198

8.1 Introduction ...................................................................................................................... 198

8.2 Structural characterisation of milled material .................................................................. 201

8.2.1 Thermal decomposition ............................................................................................. 203

8.2.2 Analysis of Decomposition Products ......................................................................... 208

8.2.3 Rehydrogenation Ability ............................................................................................ 218

8.3 Novel quaternary hydrides (MxCa1-xMgH4), (M=Li, Na) ................................................ 221

8.3.1 Quaternary Hydride (LixCa1-xMgH4) ......................................................................... 221

8.3.1.1 Structural characterisation of as milled material ................................................. 221

8.3.1.2 Thermal decomposition ....................................................................................... 224

8.3.1.3 In-situ XRD ......................................................................................................... 226

8.3.2 Quaternary Hydride (NaxCa1-xMgH4) ........................................................................ 235

8.3.2.1 Structural characterisation of as milled material ................................................. 235

8.3.2.2 Thermal decomposition ....................................................................................... 242

8.3.2.3 In-situ XRD ......................................................................................................... 245

8.4 Conclusions ...................................................................................................................... 253

8.4.1 Ca-Mg-H ternary hydride .......................................................................................... 253

8.4.2 LixCa1-xMgH4 (x=0.2) ................................................................................................ 254

8.4.3 NaxCa1-xMgH4 (x=0.2) ............................................................................................. 255

9. GENERAL DISCUSION ................................................................................................... 257

10. CONCLUSIONS & FUTURE WORK ............................................................................ 267

10.1 Conclusions .................................................................................................................... 267

LIST OF FIGURES

Figure 1.1 Comparison of specific energy (energy per mass or gravimetric density) and energy

density (energy per volume or volumetric density) for several fuels. (Schlapbach and Züttel,

2001) ...................................................................................................................................... 4

Figure 1.2 Schematic function mechanism of a PEMFC(Britannica, 2007) .............................. 7

Figure 1.3 Volumetric against gravimetric energy storage densities of different energy carriers.

Hydrogen shows very low volumetric density, but a very high gravimetric energy density

(Züttel et al., 2010) ................................................................................................................ 8

Figure 1.4 Relative volumes required to store 4 kg of hydrogen based on a 400 km driving range

using different techniques (Hanwha, 2013) ........................................................................... 9

Figure 1.5 Potential storage systems that can meet the targets for on-board light duty

vehicles((DOE), 2012b) ....................................................................................................... 11

Figure 1.6 Volumetric and Gravimetric densities of a series of investigated materials for

hydrogen storage. Modified from (Züttel, 2003). Red – metal hydrides. Purple – complex

hydrides. Green – carbon based hydrides (Züttel, 2003). Red dashed circle – materials target

............................................................................................................................................. 13

Figure 1.7 Schematic diagram of possible hydrogen bonding states in hydrogen storage

materials. (Orimo et al., 2007) ............................................................................................. 17

Figure 1.8 Pressure composition isotherm plot of LaNi5 (left), van’t Hoff plot (right) (Züttel,

2003, Guo, 2015). ................................................................................................................ 19

Figure 2.1 Schematic structural illustration of catalyst layer covered on Mg particle.

Reproduced from (Cui et al., 2013) ..................................................................................... 29

Figure 2.2 Temperature-Programmed Desorption (TPD) profiles for the ball-milled MgH2 in

Ar at a heating rate of 5 ᵒC/min, fully hydrogenated BM-R sample and BM sample.

Reproduced from (Cui et al., 2013). .................................................................................... 30

Figure 2.3 Theoretically achievable reversible storage capacities and reaction enthalpies of

selected hydrides. LaNi5H6 and FeTiH2 are taken as examples for conventional room

temperature hydrides. Reproduced from (Dornheim, 2011) ............................................... 31

Figure 2.4 Modification of the thermodynamic properties of M-H by altering the stability of the

hydrogenated or dehydrogenated state. Reproduced from (Dornheim, 2011). ................... 32

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Figure 2.5 (a) the unit-cell parameter, (b) cell volume, and (c) tilting angle for Na1-xLixMgH3

as a function of the amount of Li at the A position of the perovskite. Reproduced from

(Martínez-Coronado et al., 2012) ........................................................................................ 41

Figure 2.6 Schematic of MgH2 heat store in a pressure container. Reproduced from (Kilner et

al., 2012) .............................................................................................................................. 43

Figure 2.7 McPhy modular standard cartridge based on Mg hydrides discs. Reproduced from

(Jehan and Fruchart, 2013) .................................................................................................. 44

Figure 3.1 Experimental techniques used in the project ........................................................... 48

Figure 3.2 Schematic a) Retsch Planetary Ball Mill PM400, b) Hardened steel milling pot 250

ml, c) Hardened steel high-pressure milling pot 220 ml. (swagelok ball valve, max pressure

150 bar) ................................................................................................................................ 50

Figure 3.3 Bragg's Law reflection. Two beams with identical wavelength and phase approach a

crystalline solid and are scattered off two different atoms within it. The lower beam traverses

an extra length of 2dsinθ. Constructive interference occurs when this length is equal to an

integer multiple of the wavelength of the radiation. Dots on X-rays represent constructive

interference (Britannica, 1999). ........................................................................................... 53

Figure 3.4 Bruker D8 advanced XRD with a 9-position multi-changer sample stage, reproduced

(Hughes, 2016) ..................................................................................................................... 54

Figure 3.5 Schematic Bruker D8 advanced XRD with an Anton Paar XRK900 reactor chamber.

............................................................................................................................................. 55

Figure 3.6 Rayleigh and Raman scattering energy diagram. S0, S1, S2 are electronic energy

levels, with higher energy vibrational levels. The dashed lines represent virtual states. .... 57

Figure 3.7 (a) Laser source, (b) Renishaw inVia Raman Microscope, (c) Instec HCS621 sample

cell. Blue line: incident beam, green line: scattered beam, yellow line: remaining beam after

removal of the Rayleigh scattering. Reproduced from (Reed, 2010) .................................. 59

Figure 3.8 Schematic of scanning electron microscope (SEM) (Atteberry, 2009) .................. 60

Figure 3.9 Left: Schematic of heat flux DSC cell, Right: high pressure DSC system (Netzsch

DSC 204 HP) with input gas. Reproduced from(Guo, 2015). ............................................. 62

Figure 3.10 Schematic simplified internal view of a Netzsch TG 209 analyser. Reproduced from

(Guo, 2015) .......................................................................................................................... 63

Figure 3.11 Schematic of a Hidden Analytical HAL IV Mass spectrometer ........................... 65

Figure 4.1 XRD patterns of the as-received and 2, 5 and 10h milled MgH2 in Ar. ................. 69

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Figure 4.2 Rietveld refinement profile for the as-received MgH2, blue line shows the observed

data, the calculated data is represented by the red line and the difference is plotted underneath

shown by the grey line. Values before 20 (2θ°) did not show any representative peaks and

therefore are neglected. Goodness of fit 1.958. ................................................................... 70

Figure 4.3 Rietveld refinement profile for the 2h milled MgH2 in Ar, blue line shows the

observed data, the calculated data is represented by the red line and the difference is plotted

underneath shown by the grey line. Values before 20 (2θ°) did not show any representative

peaks and therefore are neglected. Goodness of fit 1.862 ................................................... 71









Figure 4.6 XRD reflections of as-received and 2, 5 and 10h milled MgH2 in 100 bar H2. ...... 75

Figure 4.7 Rietveld refinement profile for the 2h milled MgH2 in 100 bar H2, blue line shows

the observed data, the calculated data is represented by the red line and the difference is

plotted underneath shown by the grey line. Diffractions before 20 (2θ°) did not show any

representative peaks and therefore are neglected. Goodness of fit 1.579. ........................... 76

Figure 4.8 Rietveld refinement profile for the 5h milled MgH2 in 100 bar H2, blue line shows



representative peaks and therefore are neglected. Goodness of fit 1.622. ........................... 77

Figure 4.9 Rietveld refinement profile of the 10h milled MgH2 in 100 bar H2, blue line shows



representative peaks and therefore are neglected. Goodness of fit 1.556 ............................ 78

Figure 4.10 Lattice parameters and cell volumes of the 2, 5, 10 h MgH2 milled in Ar a) Mg, b)

γ-MgH2 and c) α-MgH2 plotted in function of the milling time. Where error bars are not

shown, they are smaller than data symbols. Solid lines are a guide for the eye. ................. 81

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Figure 4.11 Lattice parameters and cell volumes of the 2, 5, 10 h MgH2 milled in 100 bar H2 a)

Mg, b) γ-MgH2 and c) α-MgH2 plotted in function of the milling time. Where error bars are

not shown, they are smaller than data symbols. Solid lines are a guide for the eye. ........... 82

Figure 4.12 Estimated crystallite size vs milling time for MgH2 samples milled under Ar and

100 bar H2 for 2, 5 and 10 h. ................................................................................................ 83

Figure 4 13 Raman spectrum of as-received MgH2 using the 633 nm excitation laser.

Reproduced from (Reed and Book, 2011) ........................................................................... 84

Figure 4.14 Raman spectrum of the 2h milled MgH2 in Ar using the 633 nm excitation laser.

............................................................................................................................................. 85

Figure 4.15 Raman spectrum of the 2, 5 and 10h milled MgH2 in 100 bar H2 using the 633 nm

excitation laser. .................................................................................................................... 85

Figure 4.16 (a) DSC, (b) TGA measurements of as-received and 2, 5 and 10h mechanically

milled MgH2 in Ar. All measurements were performed at a heating rate of 2 °C/min under

flowing 3 bar Ar at 100 ml/min (DSC) and at 1 bar Ar flowing at 40 ml/min (TGA).

Temperatures before 250 °C do not show any representative peaks and thus, DSC and TGA

curves are plotted from 250 up to 415 °C. ........................................................................... 88

Figure 4.17 (a) DSC, (b) TGA measurements of as-received and 2, 5 and 10h mechanically

milled MgH2 in 100 bar H2. All measurements were performed at a heating rate of 2 °C/min

under flowing 3 bar Ar at 100 ml/min (DSC) and at 1 bar Ar flowing at 40 ml/min (TGA).

Temperatures before 250 °C do not show any representative peaks and thus, DSC and TGA

curves are plotted from 250 °C to 415 °C where the reactions were completed. ................ 90

Figure 4.18 Hydrogen desorption temperatures (onset, peak, final) vs milling time of

mechanically milled MgH2 under Ar for 2, 5 and 10 h. ...................................................... 91

Figure 4.19 Hydrogen desorption temperatures (onset, peak, final) vs milling time of

mechanically milled MgH2 under 100 bar H2 for 2, 5 and 10 h. .......................................... 92

Figure 4.20 In-situ XRD diffractions of the 2h milled MgH2 in Ar, heated at 2 °C/min under 3

bar He flowing at 100 ml/min. Reflections below 25 (2θ°) were not considered as no

representative diffractions were detected. ........................................................................... 94

Figure 5.1 Li-Mg alloy (ribbon) acquired from Ilika plc with approximate scale bar. .......... 100

Figure 5.2 Lithium Magnesium Alloy phase diagram (Nayeb-Hashemi et al., 1984) ........... 100

Figure 5.3 XRD pattern of as-received Li-Mg ribbon, and after ball milling for 1 h. ........... 102

Figure 5.4 Rietveld refinement of as-received Li-Mg ribbon, values before 25 were not

considered for the refinement as no representative diffractions were detected. (Observed data

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is shown by blue lines, calculated data is represented by the red lines and the difference is

plotted below in grey). Goodness of fit 2.013 ................................................................... 102

Figure 5.5 Rietveld refinement of 1 h milled in 100 bar H2 Li-Mg ribbon, values before 25

were not considered for the refinement as no representative diffractions were detected.

(Observed data is shown by blue lines, calculated data is represented by the red lines and the

difference is plotted below in grey). Goodness of fit 2.137 .............................................. 103

Figure 5.6 DSC measurements of the as received and 1h milled (100 bar H2) Li-Mg alloy (a)

under 3 bar Ar at 100ml/min and 2 °C/min heating rate, (b) under 3 bar H2 flowing at 100

ml/min and a heating rate of 2 °C/min ............................................................................... 106

Figure 5.7 TGA of the 1h milled (100 bar H2) Li-Mg sample heated under 3 bar Ar flowing at

40 ml/min, heating rate 2 °C/min....................................................................................... 107

Figure 5.8 In-Situ XRD of as-received Li-Mg ribbon heated and cooled in 3 bar He flowing

at100ml/min. No reflections were detected below 25 °C, hence are not shown. .............. 109

Figure 5.9 In-Situ XRD diffraction patterns for Li-Mg sample heated and cooled down under 3

bar H2 flowing at 100ml/min. No reflections were detected below 25 °C, therefore, are not

presented. ........................................................................................................................... 110

Figure 5.10 In-Situ XRD diffraction patterns for 1h milled (100 bar H2) Li-Mg sample heated

and cooled down under 3 bar H2 flowing at 100ml/min. No reflections were detected below

25 °C, hence, are not presented. ......................................................................................... 112

Figure 5.11 Lattice parameters a, c of the Mg phase in the 1h milled Li-Mg alloy milled in 100

bar H2. Solid lines are a guide for the eye. ........................................................................ 113

Figure 5.12 Unit cell volumes plotted against temperature for the Li-Mg, LiH, Mg and MgH2

phases in the Li-Mg alloy milled for 1h in 100 bar H2. Solid lines are a guide for the eye.

Where error bars are not shown, they are smaller than data symbols ............................... 113

Figure 5.13 Unit cell parameter and volume plotted against temperature for the LiH phase in

the Li-Mg alloy milled for 1h in 100 bar H2. Solid lines are a guide for the eye. Where error

bars are not shown, they are smaller than data symbols .................................................... 114

Figure 6.1 NaMgH3 unit cell, light grey, red, and gold spheres represent H, Mg and Na

respectively (left); Structure of NaMgH3 as a polyhedral representation viewed along the

[010] direction (right). ....................................................................................................... 118

Figure 6.2 Calculated pressure-temperature equilibrium diagram for NaMgH3 in comparison

with experimental data from the literature (Abdessameud et al., 2014). ........................... 121

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Figure 6.4 XRD patterns for the as-received NaH (top) and MgH2 (bottom) ........................ 122

Figure 6.3 DSC-TGA-MS of the as-received (left) NaH and (right) MgH2 heated at 2 °C/min in

3 bar Ar flowing at 100 ml/min (DSC), and 1 bar Ar flowing at 40 ml/min (TGA). MS shows

the hydrogen evolution of the as-received materials. ........................................................ 122

Figure 6.5 RT-XRD patterns of NaH and MgH2 mechanical milled for 2, 5 and15h in Ar

showing NaMgH3 phase. .................................................................................................... 124

Figure 6.6 RT-XRD patterns of NaH and MgH2 mechanical milled for 2, 5 and15h in 100 bar

H2 showing NaMgH3 phase formation. ............................................................................. 125

Figure 6.7 TOPAS refinement of NaH+MgH2 milled for 5h in Ar. 2 theta (°) values before 25

were not considered for the refinement as no representative diffractions were detected.

(Observed data is shown by blue lines, calculated data is represented by the red lines and the

difference is plotted below in grey). Goodness of fit 1.214 .............................................. 126

Figure 6.8 Lattice parameters a (black line), b (blue line) c (green line) and cell volume (red

line) of the NaMgH3 phases obtained from NaH+MgH2 samples that had been milled in Ar

for 2, 5 and 15h. Where error bars are not shown, they are smaller than data points. ....... 127

Figure 6.9 Lattice parameters a (black line), b (blue line) c (green line) and cell volume (red

line) of the NaMgH3 phases obtained from NaH+MgH2 samples that had been milled in 100

bar H2 for 2 and 5h. Where error bars are not shown, they are smaller than data points. . 128

Figure 6.10 (a) DSC, (b) TGA and (c) MS measurements of NaH+MgH2 which had been milled

in Ar for 2, 5, and 15h. All measurements were carried out at a heating rate of 2 °C/min

under 3 bar flowing Ar at 100 ml/min (DSC), and 1 bar Ar flowing at 40 ml/min (TGA).

Temperatures lower than 250 °C did not show any peaks, hence are not plotted. ............ 131

Figure 6.11 (a) DSC, (b) TGA and (c) MS measurements of NaH+MgH2 which had been milled

in 100 bar H2 for 2 and 5h. All measurements were carried out at a heating rate of 2 °C/min

under 3 bar flowing Ar at 100 ml/min (DSC), and 1 bar Ar flowing at 40 ml/min (TGA).

Temperatures lower than 250 °C did not show any peaks, hence are not plotted. ............ 132

Figure 6.12 In-situ XRD of NaH+ MgH2 milled for 5h in Ar heated from 30 to 400 °C at 2 °C

in 3bar He flowing at 100 ml/min ...................................................................................... 135

Figure 6.13 RT XRD of the decomposed milled NaH+MgH2 for 5h in Ar after In-Situ XRD

........................................................................................................................................... 136

Figure 6.14 Retvield refinement of the decomposed milled NaH+MgH2 for 5h in Ar after In-

Situ XRD. Red line shows the calculated data. Blue line shows the data received from the

cif file. Grey line shows the difference between the observed and calculated data. Goodness

of fit = 1.212 ...................................................................................................................... 136

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Figure 6.15 In-Situ XRD of NaH+MgH2 milled for 5h in 100 bar H2 heated from 30 to 400 °C

at 2 °C in 3bar He flowing at 100 ml/min.......................................................................... 138

Figure 6.16 Surface plot from in-situ XRD of 5h milled NaMgH3 heated under He (100 ml/min,

~3 bar) between 30 – 450 C. A brighter colour corresponds to a higher diffraction intensity.

The white dashed lines show the phase transition temperature and are given as a guide for

the eye. ............................................................................................................................... 139

Figure 6.17 In-Situ XRD of the decomposed NaH+MgH2 sample milled for 5h (Ar) in 3 bar

He. Heated from RT to 400 °C in 10 bar H2 flowing at 100 ml/min. Blue dashed lines show

the rehydrogenation of the NaMgH3 phase. ..................................................................... 142

Figure 6.18 Ex-situ XRD of the re-hydrogenated NaH+MgH2 that has been milled for 5h in Ar.

........................................................................................................................................... 143

Figure 6.19 TOPAS refinement of the re-hydrogenated NaH+MgH2 milled for 5h (Ar). 2 theta

() values before 25 were not considered for the refinement as no representative diffractions

were detected. (Observed data is shown by blue line; calculated data is represented by the

red line and the difference is plotted below in grey). Goodness of fit 1.321 ..................... 144

Figure 6.20 Surface plot from in-situ XRD of rehydrogenated NaMgH3 heated under H2 (100

ml/min, ~10 bar) between 30 – 450 C. A brighter colour corresponds to a higher diffraction

intensity. The white dashed lines show the phase transition temperature and are given as a

guide for the eye. ............................................................................................................... 145

Figure 7.1 XRD patterns of 5h ball milled LixNa1-xMgH3 (x = 0, 0.2, 0.5, 0.8) hydride system.

Main NaMgH3 peak zoomed in for the different Li x substitutions. ................................. 152

Figure 7.2 Unit cell parameters (top) and Cell Volume parameters (bottom) of LixNa1-xMgH3

hydride system in function of Li substituted into the system. This work (left) vs literature

(right)11. Where error bars are not shown, they are smaller than the data symbols. .......... 154

Figure 7.3 (a) DSC, (b) TGA and (c) MS curves of nominal composition LixNa1-xMgH3 hydride

system (x=0, 0.2, 0.5 and 0.8). All measurements were performed at a heating rate of 2

C/min from 30 to 400 C in flowing 3 bar Ar at a rate of (100 ml/min) (DSC) and 1 bar Ar

flowing 40 ml/min (TGA). Data below 200 C are not plotted as no exo/endothermic

reactions were detected. ..................................................................................................... 158

Figure 7.4 In-situ XRD of 5h ball milled LixNa1-xMgH3 (x=0) showing the decomposition

reactions of the sample under 3 bar flowing He atmosphere. Measurements were taken from

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30C to 400C at a rate of 2C/min. values before 28 were not considered as no

representative diffractions were detected. ......................................................................... 160

Figure 7.5 RT XRD of the 5 h milled LixNa1-xMgH3 (x=0) after dehydrogenation under 3 bar

He flowing at 100 ml/min. 2 theta () values before 25 were not considered as no


Figure 7.6 TOPAS refinement of LixNa1-xMgH3 (x=0), 2 theta () values before 25 were not


is shown by blue line, calculated data is represented by the red lines and the difference is


Figure 7.7 In-situ XRD of 5h ball milled Li0.2Na0.8MgH3 showing the decomposition reactions

of the sample under 3 bar flowing He atmosphere. Measurements were taken from 30C to

400C at a rate of 2C/min. values before 28 were not considered as no representative

diffractions were detected. ................................................................................................. 164

Figure 7.8 RT XRD of the 5 h milled LixNa1-xMgH3 (x=0.2) after dehydrogenation under 3 bar

He flowing at 100 ml/min. 2 theta () values before 25 were not considered as no


Figure 7.9 TOPAS refinement of LixNa1-xMgH3 (x=0.2), 2 theta () values before 25 were not




Figure 7.10 Surface plot from in-situ XRD of LixNa1-xMgH3 (x=0.2) heated under He (100



guide for the eye. ............................................................................................................... 167

Figure 7.11 In-situ XRD of 5h ball milled Li0.5Na0.5MgH3 showing the decomposition reactions




Figure 7.12 RT XRD of the 5 h milled LixNa1-xMgH3 (x=0.5) after dehydrogenation under 3

bar He flowing at 100 ml/min. 2 theta () values before 25 were not considered as no




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guide for the eye. ............................................................................................................... 172

Figure 7.15 In-Situ XRD of 5h ball milled Li0.8Na0.2MgH3 showing the decomposition reactions




Figure 7.16 RT XRD of the 5 h milled LixNa1-xMgH3 (x=0.8) after dehydrogenation under 3

bar He flowing at 100 ml/min. 2 theta () values before 25 were not considered as no





guide for the eye. ............................................................................................................... 176

Figure 7.18 In-situ XRD of the 5 h milled LixNa1-xMgH3 (x=0) rehydrogenated sample under

10 bar H2 flowing at (100 ml/min) after decomposition. Data was collected isothermally in

50 °C intervals. Dashed (black line) indicates the patterns where rehydrogenation is detected.

........................................................................................................................................... 178

Figure 7.19 RT XRD of the 5 h mi lled LixNa1-xMgH3 (x=0) after re-hydrogenation in 10 bar

H2 flowing at 100 ml/min. 2 theta () values before 25 were not considered as no


Figure 7.20 TOPAS refinement of LixNa1-xMgH3 (x=0), 2 theta () values before 25 were not




Figure 7.21 In-situ XRD of the 5 h milled LixNa1-xMgH3 (x=0.2) rehydrogenated sample under


50 C intervals. Dashed (black line) indicates the patterns where rehydrogenation is detected.

........................................................................................................................................... 182

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Figure 7.22 RT XRD of the 5 h milled LixNa1-xMgH3 (x=0.2) after re-hydrogenation in 10 bar










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Figure 8.1 a) Schematic unit cells of CaH2 and the ternary phases b) Ca19Mg8D54 and c)

Ca4Mg3D14. (Green, grey and blue spheres represent Ca, H and Mg, respectively.) ...... 200

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University of Birminghametheses.bham.ac.uk/id/eprint/8302/1/Contreras_Vasquez18PhD.pdfSYNOPSIS Synthesis, characterisation and hydrogen sorption properties of a variety of magnesium